Interrelationships between p-Coumaric Acid, Evapotranspiration, Soil Water Content, and Leaf Expansion

Increasing concentrations of p-coumaric acid applied to (cucumber seedling)–[Cecil Ap soil–sand mixture (or soil)] systems inhibited evapotranspiration (primarily transpiration) and leaf area expansion of cucumber seedlings and increased soil moisture. Higher soil moisture resulting from the inhibition of evapotranspiration lowered soil solution concentrations of p-coumaric acid by 14–40% but did not significantly influence the inhibitory effects of p-coumaric acid on seedlings. Inhibition of evapotranspiration and total leaf area and increases in lowest daily soil water were observed 1–3 d after the first p-coumaric acid treatment, whereas inhibition of absolute and relative rates of leaf expansion was observed within a 24-hr period. Development of the maximum effects of p-coumaric acid required several additional days. Recovery from effects, i.e., return to control levels, after p-coumaric acid depletion from soil solution was a gradual process requiring days for evapotranspiration, lowest daily soil water, and total leaf area, but was slightly faster for leaf area expansion. It appears, at least for short-term studies, that the initial input or treatment concentrations of p-coumaric acid represented a reasonable estimate of dose despite the dynamic nature of soil solution concentrations, and that the lowering of available p-coumaric acid concentrations, associated with the elevation of soil moisture, did not result in a concurrent detectable seedling response. However, increased soil moisture associated with p-coumaric acid treatments of sensitive species suggests a means by which the magnitude of some allelopathic interactions may be modified and resource competition and allelopathy could interact.